1. Field of the Invention
The present invention relates to an apparatus capable of compensating for the thermal displacement of the main spindle of a machine tool, wherein the final position of the main spindle is compensated for the thermal displacement of the main spindle and the spindle head.
2. Description of the Prior Art
In the past, a few kinds of compensating apparatus capable of compensating thermal displacement of a spindle head were proposed.
For example, the U.S. Pat. No. 3,045,510 discloses a compensating apparatus, wherein a detecting bar formed of temperature stable material is attached to the front end portion of a spindle head, and the rear end of the detecting bar is engaged with a differential syncro in order to detect the axial movement of the detecting bar. The output signal of the differential syncro is fed to a position control system so as to modify the final position of the spindle head. In the compensating apparatus described above, the output signal of the differential syncro is in proportion to the thermal displacement of the front end portion of the spindle head. Accordingly, the feed amount of the spindle is compensated for the thermal displacement of the spindle head so that the machining accuracy of workpieces machined with a tool carried by the main spindle is improved.
Generally a spindle head thermally extends and contracts with respect to a ball nut mounting surface at which a ball nut is attached in order to transmit the driving power generated by a feed motor. Furthermore, the main spindle also thermally extends and contracts with respect to the spindle bearing which supports the main spindle on the spindle head. Accordingly, the front end portion of the main spindle is displaced with respect to the ball nut mounting surface by the amount which is total of the thermal displacement of the spindle head and the thermal displacement of the main spindle. However, the compensating apparatus described above can compensate for only the thermal displacement of the spindle head. Accordingly, it can not improve the machining accuracy to a sufficient level.